1. Field of the Invention
The present invention relates to a vertical alignment active matrix liquid crystal display device whose liquid crystal molecules are aligned vertical to the surface of a substrate.
2. Description of the Related Art
A conventional TFT liquid crystal panel comprises a TFT (Thin Film Transistor) substrate on which TFTs and pixel electrodes and the like are formed, a CF (Color Filter) substrate on which color filters and an opposing electrode are formed, and a liquid crystal layer which is sandwiched between those substrates. A TFT liquid crystal display panel which has liquid crystal molecules homogeneously aligned, such as a TN (Twisted Nematic) liquid crystal display device, uses a material with a positive dielectric anisotropy. A TFT liquid crystal display panel which has liquid crystal molecules homeotropically aligned uses a material with a negative dielectric anisotropy, and aligns the director (molecular long axis direction) perpendicular to the substrate with no electric field formed (initial aligned state).
In the vertical alignment TFT liquid crystal display device whose liquid crystal molecules are homeotropically aligned in the initial aligned state, a vertical alignment film is formed on the inner surface of each of a pair of substrates arranged opposite to each other, and a liquid crystal with the negative dielectric anisotropy is filled between the substrates, thereby constituting a liquid crystal cell.
In the liquid crystal cell, a plurality of pixel electrodes are formed on one of the pair of substrates, and an opposing (common) electrode which faces the picture electrodes is formed on the other substrate, with one pixel being formed by each pixel electrode, the opposing portion of the common electrode, and the liquid crystal therebetween. A vertical alignment film, which is rubbed to define a direction in which the liquid crystal molecules tilt when a voltage is applied between the pixel electrode and the opposing electrode, is so formed on each substrate as to cover the pixel electrode and the opposing electrode.
In a case where no voltage is applied between the pixel electrode and the opposing electrode, because the opposing electrode and the pixel electrode have the same electric potential, no electric field is formed between those electrodes, and the action of the vertical alignment film causes the liquid crystal molecules to be aligned vertical to the substrate.
When a voltage is applied between the pixel electrode and the opposing electrode, the liquid crystal molecules behave to tilt because of the electric field formed between those electrodes. When a sufficiently high voltage is applied between the pixel electrode and the opposing electrode, the liquid crystal molecules are aligned substantially horizontal to the substrate.
In this case, when a voltage is applied between the pixel electrode and the opposing electrode, the liquid crystal molecules are aligned along one direction due to the electric field formed between those electrodes, and alignment restricting force of the alignment process. This results in a large view angle dependency of the contrast and a poor view angle characteristic.
To obtain a wide view angle characteristic in the vertical alignment liquid crystal display apparatus, it is proposed to form a plurality of domains where the liquid crystal molecules are aligned along plural directions pixel by pixel. For instance, as described in the specification of Japanese Patent Publication No. 2565639, a liquid crystal display apparatus proposed has the opposing electrode formed with an aperture with the shape of a letter X, so that the liquid crystal molecules in each pixel are so aligned as to tilt toward the center of the X-shaped aperture along the four directions when a voltage is applied between the two electrodes facing each other.
In this liquid crystal display apparatus, the opposing electrode is formed larger than the pixel electrode, and when a voltage is applied between the pixel electrode and the opposing electrode, a vertical electric field (an electric field vertical to the substrate) is generated at that portion of the pixel region where the pixel electrode faces the opposing electrode, and an oblique electric field is generated at the peripheral portion of the pixel electrode, thereby forming a discontinuous electric field portion at that portion of the opposing electrode where the slit is formed. The liquid crystal molecules are aligned to tilt toward the center of the X-shaped aperture due to the discontinuous electric field portion. That is, in this liquid crystal display apparatus, the liquid crystal molecules are so aligned as to tilt along the four directions for each pixel and for each region defined by the X-shaped aperture.
According to the above-described liquid crystal display apparatus, however, because the X-shaped aperture formed in each pixel forms regions with different alignment directions, the X-shaped aperture should be formed wide enough to prohibit the interaction between the regions. Accordingly, in each pixel, the area of the aperture (slit) which is not controllable by the electric field becomes large, and the area of the opposing electrode becomes small, resulting in a low aperture ratio.